docs: CG-21 A/B — the gate passes, all four bars (#1500)

Re-runs CG-15's agent A/B on the fixed build: same harness, same three prompts, same baseline ref, n=6 per arm on express and excalidraw. Read = 0 in all 15 new-arm runs. The express regression that routed the defect to CG-21 does not reproduce in 6 attempts, and the baseline now reads in 4 of 6 while the new arm reads in none (median 24.5s -> 21.5s), so the control beats the arm it previously lost to. client-go holds 92.7-96.2% answer share against a baseline run at 53.8%. Excalidraw's new arm is ~8s slower at the median and that is recorded as NOT attributable to the build rather than waved through: explore's own latency is 374ms vs 372ms on the same query and index, the deterministic responses differ by +2% with one byte-identical, and the unchanged main build's own median moved 34s -> 26.5s between the two sessions — the same magnitude as the gap. Bars were not re-baselined; they are CG-15's four, applied to a larger sample. The CG-15 section is kept intact and marked superseded, because its root-cause analysis is the record of why the fix looks like it does.
2026-08-04 02:40:10 -05:00
parent fca7d87047
commit abee46c5e4
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@@ -290,6 +290,7 @@ whether a change to codegraph helped, not whether codegraph helps.
 | Change | Repos | Result |
 |---|---|---|
 | **Score-proportional byte allocation** (#1500, epic CG-1) — 2026-08-04, `feature/CG-1` vs `main`, 3 runs/arm | client-go (Go, 2,454 f, 2,001 generated — the reporter's shape), excalidraw (TS, 672 f), express (JS, 147 f, control) | **Gate FAILED.** Read 0/0/0 both arms on client-go and excalidraw; excalidraw **34s → 24s median with one fewer explore call**; generated clientsets/informers drop from 10.5% of a baseline envelope to 0% in every new run. But express regressed in 1 of 3 runs (**4 Reads, 52s**) from a reproducible non-agent cause: a file whose proportional reservation lands below its own size no longer renders whole and its cluster render leaves the reservation **unspent** (`lib/utils.js` 6,380 B whole → 583 B stub, envelope 13.8K → 9.2K). Full record: [`docs/benchmarks/explore-allocation-ab-1500.md`](../benchmarks/explore-allocation-ab-1500.md) |
+| **↳ re-run after CG-21** — 2026-08-04, `feature/CG-1` @ `fca7d87` vs the same `main`, **6 runs/arm** on express + excalidraw, 3 on client-go | same three repos | **Gate PASSES, all four bars.** **Read = 0 in all 15 new-arm runs** — the express regression does not reproduce in 6 attempts, and the *baseline* now reads in 4 of 6 while the new arm reads in none; express median **24.5s → 21.5s**. client-go answer share 92.7–96.2% vs a baseline run at 53.8%. Excalidraw's new arm is ~8s slower at the median, **not attributed to the build**: explore's own latency is 374 ms vs 372 ms (n=5), deterministic responses differ by +2% with one byte-identical, and the *unchanged* `main` build's own median moved 34s → 26.5s between the two sessions. Deterministic core: `lib/utils.js` 583 B stub → **6,268 B whole**, envelope 9.2K → **14.5K** on an unchanged budget |

 Two harness lessons from that run, both now baked into `ab-new-vs-baseline.sh`:

@@ -616,3 +616,41 @@ Two traps the first drafts fell into, both of which made a test pass on the defe
  legitimately under-spends it (the fixture's `response.ts`: 1,635 delivered of 5,292 reserved).
  The assertion is per-file — *delivered >= min(reservation, file size)* — which is what express's
  `utils.js` violated and a pool-sum assertion does not express.
+
+### A third condition, found by review rather than by a test
+
+A buy must also **fit the render ceiling**. The whole-file branch refuses to slice a file
+mid-method, so a whole render that overruns `renderCeiling` is skipped *entirely* — meaning a
+buy approved by the funding pool but refused by the ceiling trades a clustered section for **no
+section**. That is the same trade the funding pool exists to refuse, arriving by another route.
+
+It is reachable only on the 24K tiers, which is why neither new fixture can see it:
+
+| tier | envelope | `renderCeiling` = `min(1.5x, 25000) - 600` | funding line = `reservedTotal + 0.15x` |
+|---|---|---|---|
+| small | 13,000 | 18,900 | ~14,350 — cannot cross |
+| medium/large | 24,000 | **24,400** | ~27,200 when saturated — **crosses by ~2.8K** |
+
+So `buysWhole` carries `totalChars + size + FILE_OVERHEAD <= renderCeiling` as well; failing it
+drops through to the cluster path, which is bounded by `headroom` and always renders something.
+The grace arm is deliberately untouched — a file within a sliver of its reservation that still
+does not fit is genuinely at the end of a full response, and that behaviour predates the epic.
+Verified inert on all three A/B repos (excalidraw and client-go byte-identical across 3 queries
+each, express reproducer unchanged), so it did not invalidate the measurement below.
+
+### The agent A/B (CG-15's gate, re-run)
+
+Full record: [`../benchmarks/explore-allocation-ab-1500.md`](../benchmarks/explore-allocation-ab-1500.md)
+§ "Re-run after CG-21". **All four bars pass**, at n=6 per arm on express and excalidraw:
+
+- **Read = 0 in all 15 new-arm runs.** The express regression that routed the defect here (4
+  Reads of `lib/utils.js`) does not reproduce in 6 attempts — and the *baseline* reads in 4 of
+  6, so the control now beats the arm it previously lost to. Median 24.5s → 21.5s.
+- **client-go** — the reporter's shape — holds 92.7–96.2% answer share against a baseline run
+  at 53.8%.
+- **Excalidraw's ~8s median gap is not attributable to the change.** Explore's own latency is
+  374 ms vs 372 ms (n=5, same query and index); deterministic responses differ by +2% with one
+  byte-identical; and the *unchanged* `main` build's own median moved 34s → 26.5s between the
+  CG-15 session and this one — the same magnitude as the gap. Agent wall-clock on this repo is
+  noise-dominated at this sample size, which is the known shape (host-model thinking dominates,
+  not tool latency).